Transubstantiation
by TheXGrayXLady
Summary: There's still one thing Byleth hasn't tried. He doesn't know if it will work. He's not sure if he can still change or if he can match Seiros in strength. If he can, there's no turning back. But Byleth has given everything she has. Her magic, her blades, and he suspects in the end, herself. He still has one last thing he can give.


**Notes: **I'm sorry, but Seteth has a very similar name to a much abused OC of mine. I can't help the angst. Warning for body horror and gore.

_**Transubstantiation**_

A demonic bird circles overhead. Nemain shrieks and tugs against her reins, nearly jerking Seteth off the ground. She snaps in frustration as he checks her tack a second time. There isn't enough time for it, but there isn't time to get it wrong either. Around him, knights and former students alike scramble to prepare to drive back the Immaculate One's horde.

In the middle of it, Byleth barks orders, rage and frustration slipping through her usual calm. He leads his agitated mount through the chaos to reach her. By then, she's drawn away from the main army.

Atop the cathedral, Seiros spits hoarfrost and roars in rage and pain. And Byleth watches, her eyes bright and calculating.

"Flyers are ready on your order," he says.

"Good," she says. Fire spells dance on her fingertips and her face is impassive as ever. "Then let's begin."

His foot's half way in the stirrup when the moment skips and Byleth falls to her hands and knees.

"I can't do it again," she says, wiping blood from her lips. Is it just the light or does her hair look darker? "Whatever power I had, it's gone."

He helps her up off the flagstones and holds her close. Her calloused, scarred fingers caress not yet wounds on his chest and one hand slips around his back, coming to rest on an unbroken spine. He wonders how many times she's watched them all die. How many times did she decide that the price of victory was too steep? How many times did she decide it wasn't? How many times had she been unable to end things alone?

"I've tried everything," she murmurs. Her voice is hoarse and numb from screaming. He hasn't heard her this flat since she came to the monastery. Only now there's no dry teasing or irreverent quips to let him know that Byleth is still in there. "I kept thinking that I just need one more chance and I'd figure it out, but nothing works. My magic, my blades, I used that damned bone sword. I even let Sothis take over and it still wasn't enough."

The more Byleth realized her emotions, the more she insisted that she was her own person. He didn't think he'd ever forgive himself for the period where he let Rhea convince him otherwise. Even then, he was relieved when sitting on Sothis' throne seemed to awaken nothing more than irritation in the professor.

Whatever happened in her futures, it must have been dire for her to resort to that.

"It has to be perfect this time," she says, her back stiffening. Her sharp, no nonsense tone sounded a little more like herself but not enough for his liking. "I have to come up with something."

There's one thing she hasn't tried. He doesn't know if it will work. He's not sure if he can still change or if he can match Seiros in strength. If it does, there's no turning back. But Byleth has given everything she has and then some. Her magic, her blades, and he suspects in the end, herself. He still has one last thing he can give.

He pulls away. "Tell Flayn I'm sorry."

Seiros cries out to her creations. The cardinals sing back to her in an awful mockery of Zanado. He goes to her, calling on old power and what was once as solid and steady as the earth under his feet is slippery and distant. Yet it's still there.

It starts in his hands. Blood drips from torn nail beds and bones crack and reform. He grits lengthening teeth. It's nothing he hasn't felt before.

"Seteth wait." The sound of rushing footsteps on cobblestones nearly breaks his resolve. He steels himself against Byleth telling him that there has to be another way.

She snatches his cloak and spins him into her. His sore back protests, but for the moments he's in her arms, it doesn't matter. She grabs his collar and her lips crash into his, mindless of erupting fangs. She takes one of his deforming hands and presses something small and warm into his palm. He winces as she closes breaking fingers around a ring.

"I love you," she says when she pulls away, her darkened eyes blazing and her face smeared with his blood. He wants to reply in kind, but the words shred on too sharp teeth. His vision's sharper now, but even if it wasn't, he couldn't miss the sadness and regret behind her rage that Rhea will take something else from her. Then she lets him go.

He staggers forwards, his back spasming with every step. Patches of earth brown and forest green spread across too smooth skin. His stomach twists against the swell of new organs and he gags on blood and bile. Mindless of how easily his claws pierce his armor and a still too soft patch of skin, he clutches his belly. Had it always hurt this much? He hopes that one pain will drown out the others, but nothing does. It's all too much, all at once.

His knees nearly go out from under him, but he holds onto his legs until he makes it to the courtyard. Then they buckle and snap, white hot pain shooting through him as the bones shift and grow. He doesn't feel it as he crashes against cool tiles.

The sharp edges of the ring bite into his scaly palm and for a moment, he's terrified he'll crush it. Had this been so hard for Seiros? It looked so seamless for her. Maybe it'd been too long since he'd last changed, or perhaps he had too many reasons to cling to human form. Flayn. Byleth. The others.

It doesn't matter. There's no stopping it. His body suddenly feels too small, too frail. He curls in on himself on the stones when his skull finally cracks. His mouth gapes, fresh blood spattering the ground with every breath.

Horns tear through his scalp and shatter his circlet. He flexes his shoulders once, twice, like a wyvern stabled for far too long. Still, the muscles are too tight. It's almost a relief when they finally tear in the frantic beating of not yet wings.

Twin ridges erupt along his spine. Thickened, leathery skin doesn't give fast enough and bone spears pierce growing lungs before ripping through his back. He chitters feebly, the sound airy and crackling.

He wants it all to stop. He's almost more afraid of when it does. There's no coming back when it does. But everyone he's holding onto humanity for, he can't protect them like this. So at long last, just before the pain is too much, Seteth gives in and slowly, so damn slowly, the pain fades. Then the change is welcoming and familiar, like coming home.

He gets to his feet, claws digging through stone, and shakes the last of the viscera from his wings. He tips his head back and the roar that splits the twilight sounds both too human and not. A blood slick ring falls from clenched claws as for the first time in a millennium, Cichol flies.


End file.
